There are a variety of applications for safety relief valves in the commercial as well as residential environment. To insure safety, the American Society of Mechanical Engineers (ASME) has prepared a code of minimum requirements for broad classifications of uses for pressure relief valves. Because of the great damage that can result from a faulty safety relief valve on a power boiler, the ASME Code Section I is the most stringent. It requires that a safety relief valve for a power boiler: 1) close at a closing pressure no lower than 96% of the set pressure for that valve; and 2) have the valve attain a full rated lift at a pressure no higher than 103% of the set pressure. In contrast, for an unfired pressure vessel, the ASME Code Section VIII requires a pressure relief valve have a closing pressure not lower than 93% of the set pressure, and a full rated lift at a pressure not to exceed the set pressure by more than 10%.
Optimally, a valve should open when the system pressure exceeds a predetermined set point for pressure and quickly reach a full open or "full lift" position without excessive overpressure buildup. Also, the valve should close as soon as it has vented the excess volume that is sufficient to return the system pressure below the set pressure and provide a rapid closing or "blowdown" to prevent an excessive loss of fluid from the system.
Attempts have been made at providing a pressure relief valve that will meet the demanding standards of ASME Code Section I. Stewart et al, U.S. Pat. No. 4,130,130 disclosed a safety relief valve that included a nozzle ring and a curved inwardly facing surface formed on a shroud portion of the valve head to define an annular secondary flow orifice. This orifice was too large to provide any restriction to flow and therefore failed to assist in the lifting force on the valve to achieve full opening and flow.
Scallan U.S. Pat. No. 4,708,164 addressed the problem of valve "chatter". The Scallan patent disclosed a safety relief valve having an adjusting ring and a valve carrier with a downwardly projecting peripheral flange. The ring included an axially projecting annular portion that extended towards the valve carrier to form an orifice to control the closing or "blowdown" of the valve. The outer peripheral surface of the ring formed a second orifice with the flange to aid in the lifting of the valve to a full open position. Though this invention provided stability in opening and closing of the valve, the closure of this valve did not meet the minimum required standard for a power boiler as dictated by ASME Code Section I.